update eLabFTW example

[NicolasCARPi]

• flatten the json-ld
• add missing mandatory properties for ./ node -> now valid ro-crate

[nicobrandt]

There are a bunch of variableMeasured with value null, I think they should be removed. The elabftw_metadata in variableMeasured is also missing a @type. Seems good to me otherwise.

[SteffenBrinckmann]

When I open it with the rocrate lib that @salexan2001 mentioned, I get an error:
ValueError: no @id in {'propertyID': 'elabftw_metadata', 'description': 'eLabFTW metadata JSON as string', 'value': '{"extra_fields": {"Raw data URL": {"type": "url", "value": "https://datalake.example.com/experiments/3921"}, "Annie, are you okay": {"type": "checkbox", "value": "", "description": "This is a checkbox custom input"}, "This is a custom list input": {"type": "select", "value": "Some choice", "options": ["Some choice", "Another choice", "A third choice"], "description": "The value is selected from a pre-defined list"}}}'}

Which are to my-understanding are not flattened.